1. Field on the Invention
This invention relates to ultraviolet curable composition which is useful as a buffer coating for optical fibers, i.e. a primary coating.
2. Background Information
The transmission of communications by means of optical fibers is commercially important today. This form of transmission is done by sending a beam of light through an optically clear fiber. Because interference with the light beam or its partial loss during the transmission must be at a minimum to make the use of optical fibers a successful communications technology, the optical fibers must be protected from any environment which will cause loss of signal or distortion of the signal. Coating the fibers is one such technique. The optical fibers are coated to protect the fiber surface from damage which can result from abrasion or water, to maintain or increase the fiber strength, and to prevent transmission loss resulting from microbending which can result from mechanical manipulation or changes in temperature. Coating materials which will provide cured films on the optical fiber which has all of these properties is difficult to achieve because improving one property often results in the decrease in another property. Optical fibers are now usually coated with at least two coatings, i.e. a primary coating or buffer coating which is applied immediately after the fiber is formed and a secondary high modulus coating which is put over the buffer coating to further protect the optical fiber. In order for the loss in transmission to be as low as possible, the buffer coat should maintain its flexible properties over a broad temperature range. Especially important is the low temperature flexibility. The low temperature flexibility can be obtained if the coating has a low glass transition temperature, T.sub.g.
Buffer coatings, useful in the optical fiber industry that protect the glass fiber from stress and microbending losses, which has the characteristic of low glass transition temperature (T.sub.g) has been a goal for sometime. Organic coatings of the prior art have difficulty achieving films which have a T.sub.g sufficiently low to be useful as a buffer coating while maintaining the rapid ultraviolet radiation (UV) cure speeds, low modulus, and physical properties needed in the optical fiber industry. The coatings for optical fibers, first used, were silicone oils, cellulosic lacquers, blocked urethanes, and room temperature vulcanizable silicones. Problems exhibited by these coating materials were handling, stability, durability, and application speed. Such problems can be overcome by UV curing. Except for the room temperature vulcanizable silicones, none of these coating materials provided low temperature flexibility down to temperatures of -40.degree. C. to -60.degree. C.
Ultraviolet radiation curable compositions are known in the art including those which are based on diacrylate-terminated polyurethane oligomers. Compositions made from these oligomers produce relatively hard films when cured. When the diacrylated-terminated polyurethane oligomers are diluted with a radiation-curable monomer having a low T.sub.g, a large amount of the monomer is necessary to achieve a soft cured material, however, the soft cured material has little strength and little utility as described by Ansel in U.S. Pat. No. 4,624,994, issued Nov. 25, 1986 on application Ser. No. 170,148, filed July 18, 1980. Ansel teaches that to make a radiation curable liquid coating which is both soft and tough, one must make an oligomer which is made up of polyurethane, polyamide, or polyurea having a molecular weight of 2000 to 8000, one amide, urea, or urethane group for every 300 to 900 units of weight, and polyalkylene polyether, polyalkylene polysulfide, or polyalkylene polyester groups. Ansel teaches how such oligomers are made. Compositions based on these oligomers constitute 20 to 50 weight percent of a radiation-curable monoethylenically unsaturated monomer having a T.sub.g below about 10.degree. C. to provide the softness, and 2 to 20 weight percent of a monoethylenically unsaturated monomer which has a strong capacity for hydrogen bonding to provide the desired toughness.
Although Ansel in U.S. Pat. No. 4,624,994 (Ser. No. 170,148) teaches UV curable compositions with T.sub.g below 10.degree. C. the properties begin to fall off when exceptionally low temperatures are encountered and it is clearly desirable to extend the range of low temperature flexibility to lower temperatures without exhibiting unacceptable stiffness. Ansel et al teach this in U.S. Pat. No. 4,496,210, issued Jan. 29, 1985. based on application Ser. No. 398,161, filed July 19, 1982, Ansel et al teach that coating materials with low modulus at low temperature such as -60.degree. C. can be obtained by compositions containing a liquid, polyethylenically unsaturated, radiation-curable resin which is an organic polysiloxane having 2 to 6 reactive side chains each of which carry a functional group including acrylic and methacrylic groups. Such silicone carbinol-based polyurethane diacrylates are polysiloxanes which have at least one monoethylenically unsaturated group for every 500 to 5,000 units of molecular weight.
Bishop et al in U.S. Pat. No. 4,472,019, issued Sept. 18, 1984, describe a buffer coating made as described by Ansel. The buffer coating is made by reacting 4 moles of 4,4'-methylene bis(cyclohexyl isocyanate) with 2 moles of polyoxypropylene glycol of molecular weight 1000 and then reacting with 2 moles of 2-hydroxyethyl acrylate and then with one mole of polyoxypropylene diamine of molecular weight of 230 in the presence of 3,4 moles of N-vinyl pyrrolidone and 917 moles of phenoxyethyl acrylate. This mixture so made is mixed with 3% by weight of diethoxy acetophenone to make it UV curable.
Bishop et al teach in U.S. Pat. No. 4,514,037, issued April 30, 1985, buffer coatings characterized by a relatively low tensile modulus at room temperature, below 10,000 psi (69 MPa), preferably below 2,000 psi (13.8 MPa). Bishop states that very low modulus ultraviolet-cured coatings are described in R. Ansel, Ser. No. 170,148, filed July 18, 1980, Bishop et al describes these coatings as follows: Ethylenic-terminated polyurethane, polyamide or polyurea oligomers having a molecular weight in the range of about 2000 to about 8000 and containing one amide, urea or urethane group for every 300 to 900 units of molecular weight, and containing polyalkylene polyether. polyalkylene polythiol or polyalkylene polyester in which the alkylene moiety contains 2 to 6 carbon atoms. These diacrylate oligomers are combined with 20% to 50% of the composition of a radiation-curable monoethylenic monomer having a T.sub.g below about -10.degree. C. such as phenoxyethyl acrylate, and with 2% to 20% of a monoethylenic monomer having a strong capacity for hydrogen bonding, such as N-vinyl pyrrolidone. Bishop et al also describe this buffer coating in U.S. Pat. No. 4,522,465, issued June 11, 1985.
Krajewski in U.S. Pat. No. 4,572,610, issued Feb. 25, 1986, filed May 21, 1984, teaches that radiation-curable coatings, and especially UV-cured coatings, are particularly desirable for the coating of optical fibers because they can be rapidly applied and rapidly cured on the freshly drawn fiber as part of the fiber's production and before the fiber can be abraded by contact with other fibers. Krajewski teaches that these coatings present a considerable problem in the area of low temperature properties. The first radiation-curable coatings were too hard as shown by the fact that once the service temperature was reduced to below room temperature, stress induced microbending was encountered. Such microbends impair the ability of the fibers to act as a wave guide at even moderately low service temperatures. Krajewski teaches that Ansel (Ser. No. 170,148) taught an acrylate polyurethane-type coating system which cured reasonably rapid with UV, had good adhesion to glass fiber, and had a low tensile modulus such that reasonable microbending resistance at moderately low temperature was achieved. Krajewski also teaches that the Ansel compositions have too high a tensile modulus to adequately avoid microbending problems under severe service conditions, down to about -60.degree. C. However, Ansel et al (Ser. No. 398,161) made progress in this direction by providing compositions which would be soft enough to resist microbending at these very low service temperatures while still maintaining minimal physical integrity at room temperature. These compositions by Ansel et al are the silicone carbinol-based polyurethane diacrylates described above. These diacrylates do not have a refractive index above 1.48 so that their utility is limited. To achieve the combination of low temperature flexibility and high refractive index acrylate-containing polybutadienes have been tried. However, the desirable characteristics do not remain constant with temperature and upon heat aging they progressively harden and the refractive index becomes lower. With this background, Krajewski describes his invention which is a radiation-curable diethylenically unsaturated polyurethane resin which has a Tg below about 20.degree. C., preferably below about 0.degree. C. is a solid, non-flowable cross-linked film at room temperature with a low tensile modulus at low temperature, i.e. less than about 10000 psi (69 MPa) at -40.degree. C., preferably less than about 5000 psi (34.5 MPa) at -60.degree. C. and a refractive index above 1.48. This urethane resin of Krajewski's is made by reacting a dihydroxy-terminated liquid polybutadiene, in which the unsaturation has been reacted with halogen, with organic isocyanate and a monoethylenically unsaturated monomer carrying a single hydroxy group to form a diethylenic polyurethane having a halogen-containing essentially saturated polybutadiene backbone.
These patents describe many ways of improving the buffer or primary coating for optical fiber. However, there still remains the need for an optical fiber buffer coating material which has an improved overall property profile. The problem illustrated by art shows that when one improves one property, it most often results in a negative improvement of another property.